Texworks is a TeX editor, not a TeX distribution (as far as I know). The way to manage packages will depend on which TeX distribution you are using / how you installed TeX. The most common are installing TeX Live either through Ubuntu's package manager (apt-get or a GUI version of it) or manually (tlmgr).
apt-get
If you installed TeX through Ubuntu's package manager, everything will be handled through it too. Look for packages called texlive-..., i.e., to install pstricks, use the command
apt-get install texlive-pstricks
This way of installing teX on Ubuntu is common as it easy, but the drawback is that packages can be (significantly) outdated, as updates are infrequent.
tlmgr
Installing TeXLive directly (called a "vanilla" installation) is a little more hassle, but you get the benefits of an up-to-date installation.
There is already a howto on TeX.sx on how to perform such an installation: How to install "vanilla" TeXLive on Debian or Ubuntu?
As for how to manage packages, you will then be using a tool called tlmgr. The full documentation is available online, and the short answer is that you install packages with
tlmgr install <package name>
and update the whole system with
tlmgr update -all
LaTeX distributions come as MiKTeX (Windows), TeX Live (cross-platform) and MacTeX flavours for different operating systems, where MacTeX is an Mac-oriented complete TeX Live together with an TeX editor and some extra-tools. Similarly proTeXt is an easy-to-install MiKTeX especially for newcomers, again together with an editor and some other tools.
None of them installs the complete CTAN resource for different reasons: platform specific programs, license issues etc.
Even if you plan to install on a computer regularly used offline it is strongly recommended to make updates at least in fairly long timeframes. The best would be a temporary internet connection, but at least for MiKteX you can choose updates from a local repository, which also can be a portable harddrive, a USB stick or a DVD.
MiKTeX/proTeXt
1. Online (pc with internet)
In MiKTeX download page, go to Other Downloads section instead of recommended download, then choose "This installer allows you download all packages and install a complete TeX/LaTeX system" depending on windows 32-bit/64-bit. (Note: 32-bit installer works on both, but 64-bit installer only on 64-bit windows.)
Both are net installers: During installation you will need internet connection, because needed packages are only downloaded just before they are installed.
2. Offline (install on pc without internet, but other computer has connection)
Your working computer will be regularly offline, but you have access to another computer with internet connection:
On that computer download the whole MiKTeX repository to a local folder. The best would be putting this folder on a removable disk, but later burning of an DVD-RW is possible as well. The download can be done with the net installer, if you change the installation setting from default to "Download MikTeX" and select then "Complete MikTeX". After choosing the nearest CTAN mirror and download folder the download starts. It can take several hours depending on the connection speed.
Later for performing updates of this repository rsync is highly recommended, see rsync in the English Wikipedia for software suggestions, or at least wget with its timestamp option.
When you start installation with the net installer on your offline pc, choose first installation from a local repository and select then the folder of the portable drive or DVD (especially do not choose “CD/DVD”, that’s only for shipped DVD’s from the developer). Attention, this path is saved in MiKTeX settings!
You should later perform updates for MiKTeX on your offline-pc on a regular basis for bugfixes and new packages.
If also on computer with internet connection MiKTeX is installed, updates should be done with the local repository, too. This saves a lot of bandwidth.
3. Offline (install on pc without internet and no internet computer nearby)
At least you need internet connection to download installers and updates.
TeX Collection DVD is your way to go. As unfortunately MiKTeX on DVD is “currently not available” try ordering the TeX Collection DVD for 2012 Online at TUG store or at TUG usergroup related to your country. It has all TeX collection.
Note to OP: If you are really interested in MiKTeX, contact its author and place a DVD feature request. Hope for the best.
Alternatively download the self-extracting proTeXt installer (over 1GB) and install.
You should notice, that updates are very likely necessary, though – so you should from time to time connect this pc to the internet (maybe in a fairly longer timeframe) or use the method described above in point 2.
For TeX Live only: Download single TeX Live 2012 ISO image file
Download 2.3 GB .iso image file via TUG: Either texlive2012.torrent or directly texlive2012.iso from one of the CTAN mirrors. Torrent is recommended for fast download on slow speed internet.
Later burn the .iso on DVD to install on an offline-pc or copy texlive.iso to USB drive and mount the texlive.iso on local pc to see the install-tl installer script.
To install for all users, you need to right-click install-tl.bat (Windows) or install-tl.sh (Linux and MacOS X) and select ‘Run as administrator’. You will see TeX Live Lion Installer window as in Screenshot-guide. Also related: TeX Live Windows installation, especially section "Troubeshooting".
Best Answer
install-tl-windows.exefrom https://www.tug.org/texlive/acquire-netinstall.html.texlivefolder on a USB stick.